This proposal is concerned with the mechanisms responsible for the conversion of medial lemniscal input into thalamocortical relay cell output in the ventral posterolateral (VPL) nucleus of the cat thalamus. Such mechanisms form one of the fundamental bases for the perception by higher centers of the qualities of amplitude and velocity of a peripheral stimulus, qualities that are not coded for directly in the place and modality properties of VPL neurons, as customarily investigated but which become first affected in lesions of the central somatosensory systems. The specific aims are: (i) to characterize the relay and gating functions of VPL neurons receiving inputs from different classes of cutaneous mechanoreceptors; (ii) to correlate differences in VPL relay neuron behavior under afferent driving with the numbers and distributions of lemniscal and GABA containing synapses; (iii) to determine the role of GABA in VPL relay cell function and the differential roles of intrinsic and reticular nucleus GABA neurons. Four primary sets of experiments are to be conducted: (a) a rigorous extra- and intracellular physiological study of cutaneous mechanoreceptor inputs to identified VPL relay neurons, using quantitatively controlled stimuli; (b) a quantitative electron microscopic study of the synaptic relations of physiologically characterized and intracellularly stained VPL cells and medial lemniscal axons; (c) an iontophoretic study of the effects of GABA, and bicuculline methiodide on VPL relay cell function; (d) a study of the effects of reversible blockade of the reticular nucleus by injections of cobalt ions on VPL relay cell function.